1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device and to a method for producing the same. More particularly, the present invention relates to a Group III nitride semiconductor light-emitting device in which a flat semiconductor layer is formed on a sapphire substrate having an uneven shape and to a method for producing the same.
2. Background Art
In the Group III nitride semiconductor light-emitting device, light may be reflected to the semiconductor layer side at the interface between the semiconductor layer and the atmospheric layer. The refractive index of GaN is 2.3 (blue LED), whereas that of air is 1, and there is a large difference. To improve the light extraction efficiency, a sapphire substrate having an uneven shape on the main surface may be used for the Group III nitride semiconductor light-emitting device. In such a semiconductor light-emitting device, light is scattered by the uneven shape, and the light extraction efficiency is high.
Japanese Patent Application Laid-Open (kokai) No. 2011-129718 discloses a Group III nitride semiconductor light-emitting device having a substrate provided with protrusions. Light which is totally reflected at the interface between the substrate and an n-type semiconductor layer or an interface between the p-electrode and the atmospheric layer and propagates in the lateral direction, is scattered by the projections, and thereby improving the light extraction efficiency.
The present inventors found that when a semiconductor layer is formed by vapor phase epitaxy such as Metal Organic Chemical Vapor Deposition (MOCVD), the following problem may arise.
When the sapphire substrate has no uneven shape, as shown in FIG. 1, a raw material gas is almost uniformly sprayed to the entire surface of the sapphire substrate. On the contrary, when the sapphire substrate has an uneven shape, as shown in FIG. 2, a raw material gas enters into the dents of the uneven shape. Therefore, in a region R1 of each dent, the concentration of the raw material gas is higher than when the sapphire substrate has no uneven shape.
When the concentration of the raw material gas is high, a semiconductor layer is easy to grow obliquely on the buffer layer on the inclined surface of the uneven shape. Particularly when a {1, 1, −2, x} plane inclined to the a-plane of sapphire exists on the inclined surface, the semiconductor layer is easy to grown on that inclined surface. This is because GaN is easy to grow on a {1, 1, −2, 0} plane. The degree of growth of semiconductor on the inclined surface of the substrate is sometimes higher than that on the main surface of the substrate. Particularly when the bottom area is small on the sapphire substrate, the degree of growth of semiconductor on the inclined surface is remarkably high. The crystal orientation of the semiconductor layer grown on the inclined surface is different from that of the semiconductor layer grown on the bottom surface. When these semiconductor layers having different growth mode are merged, the surface of the growth layer after merger is difficult to be flat. Moreover, the crystallinity of the growth layer after merger is deteriorated.
When the uneven shape is formed in high density, that is, the pitch width of the tops of the adjacent mesas is small, the light extraction efficiency is improved. However, the smaller the pitch width, the more difficult the surface of the base layer is to be flat.